1. Field of the Invention
The present invention relates to a method of forming a resist (mask) pattern of organic material in a lithographic process, and more particularly, to a multilevel resist process for accurately forming a resist pattern, which pattern is used in the fine patterning of semiconductor devices (e.g., ICs) during fabrication.
2. Description of the Related Art
Resist patterns are indispensable to the fabrication of semiconductor devices, such as LSI and VLSI, and in particular, to the patterning (selective etching) of layers such as insulating layers, conductive (e.g., metal) layers, and a semiconductor layer. Recent demands for a further miniaturization of the semiconductor elements of the semiconductor device have brought a corresponding demand for finer resist patterns, for example, a resist pattern having a minimum width of less than 1 .mu.m, i.e., a submicron pattern.
Where a multilayer structure is formed on a semiconductor substrate during the fabrication of a semiconductor device, an insulating layer or metal layer has steps having a height of approximately 1 to 2 .mu.m, and in this case, it is very difficult to accurately form a fine pattern on the insulating or metal layer by a single-layer resist process. Therefore, to accurately form a fine pattern on a stepped layer, multilevel resist processes, including a trilevel resist process and a bilevel resist process, have been proposed.
A conventional trilevel resist process is now explained with reference to FIGS. 1A to 1D and FIG. 2.
A resist (mask) pattern is formed in accordance with a conventional trilevel resist process in the following manner. As shown in FIG. 1A, on a layer 1, which is to be selectively etched (patterned) and has a step caused by an underlying layer (not shown) formed on a semiconductor layer, and which is, e.g., an aluminum (Al) layer, a planarizing layer (thick lower layer) 2 of organic material is formed to cover the step and provide a flat surface thereon. The planarizing layer 2 has a thickness greater than the height of the step, and the organic material is a resin which can be easily etched by oxygen plasma, such as phenolic novolak and cresylic novolak, and may or may not be photosensitive. Then an intermediate (isolation) layer 3 having a high resistance to oxygen dry etching is formed on the planarizing layer 2. The intermediate layer 3 is made of, e.g., silicon oxide (SiO.sub.2), silicon nitride (Si.sub.3 N.sub.4) or the like, and thereafter, a thin imaging resist layer (upper layer) 4 for pattern formation is formed on the intermediate layer 3.
As shown in FIG. 1B, the resist layer 4 is exposed and developed by using an optical, electron-beam or X-ray lithography process to shape the layer 4 into a predetermined pattern, and since the resist layer 4 is thin, a high resolution pattern of an exposure technique used can be obtained. Then, using the upper resist layer 4 as a mask as shown in FIG. 1C, the intermediate layer 3 is selectively etched by a dry anisotropic etching method, such as a reactive-ion etching (RIE) method, to shape the layer 3 into the predetermined pattern. Next, as shown in FIG. 10, using the intermediate layer 3 as a mask, the planarizing lower layer 2 is selectively etched by a dry anisotropic etching method using an oxygen etching gas, e.g., an RIE process under oxygen plasma, to shape the layer 2 into the predetermined pattern. In practice, however, an undercutting having a width "l" appears under the edge portion of the intermediate layer 3, as shown in FIG. 2, since not only a main vertical etching component but also an oblique etching component are generated, and the latter causes side-etching. Accordingly, the dimensions of the obtained resist mask pattern are not aligned with the originally predetermined pattern dimensions. When the layer (Al layer) 1 to be etched is dry-etched by using the obtained planarizing lower layer 2 as a mask, the layer 1 is patterned in accordance with the obtained lower layer 2 having the misaligned dimension, and therefore, the obtained resist mask pattern cannot provide a satisfactorily fine and accurate patterning.
A resist (mask) pattern is formed by a conventional bilevel resist process in the following manner. As shown in FIG. 3A, on a stepped layer (e.g., Al layer) 1 to be selectively etched, a planarizing layer (thick lower layer) 2 of the above-mentioned organic material is formed to cover the step and to form a flat surface. Then an upper resist layer 8 serving as the intermediate layer and the upper resist layer used in the trilevel resist process is formed on the planarizing lower layer 2. The upper resist layer 8 is of, e.g., photosensitive silicon-containing resin. Then, as shown in FIG. 3B, the upper resist layer 8 is exposed and developed by a suitable lithography technique to shape the layer 8 into a predetermined pattern. Next, as shown in FIG. 3C, using the upper resist layer 8 as a mask, the planarizing lower layer 2 is selectively etched (patterned) by a dry anisotropic plasma RIE process, to shape the layer 2 into the predetermined pattern. In practice, however, as shown in FIG. 4, an undercutting having a width "l" appears in the same manner as in the above-mentioned trilevel resist process case, and thus the dimensions of the obtained resist mask pattern are not aligned with the originally predetermined dimension. Therefore, the obtained planarizing lower layer 2 cannot provide a satisfactorily fine and accurate patterning.
For example, a planarizing lower layer of novolak (OFPR-800: trade name of product manufactured by Tokyo-Ohka Kogyo Kabushiki Kaisha, Japan), an intermediate layer of SiO.sub.2 and an upper resist layer of the same novolak were, in sequence, formed on a substrate to form a trilevel resist structure. After the exposure and development of the upper layer, and selective etching of the intermediate layer, the lower layer was selectively etched (patterned) under a generation of oxygen plasma in a parallel-plate type RIE apparatus using oxygen gas as an etchant gas, and the results shown in FIG. 5 were obtained. As can be seen from FIG. 5, the lower the oxygen gas pressure, the smaller the undercutting width "l"; but the undercutting does not disappear. On the other hand, as the oxygen gas pressure is decreased, the etching rate of the lower layer is increased to a peak value and then decreased to a relatively small value.
Instead of the RIE method, another plasma etching method utilizing an electron cyclotron resonance (ECR) effect can be used. The ECR etching method is performed at an oxygen gas pressure lower than that of the RIE method, and carries out a dry etching having a directionality stronger that of the RIE anisotropic etching. Accordingly, the ECR etching method carries out an anisotropic etching with less undercutting. The above-mentioned planarizing lower layer was selectively etched under a generation of oxygen plasma in an ECR etching apparatus using oxygen gas, and the results shown in FIG. 6 were obtained. In FIG. 6, black circles " " and black triangles " " indicate the under-cut width and etching rate, respectively, under a condition that a high-frequency bias is applied to a holder for substrates of the ECR etching apparatus, and white circles ".smallcircle." and white triangles ".DELTA." indicate the undercutting width and etching rate, respectively, when a bias is not applied to the holder. As can be seen from FIG. 6, the bias condition is preferable to the non-bias condition, and a lower oxygen gas pressure (i.e., vacuum pressure) reduces the under-cutting width. Namely, at a pressure of 3.times.10.sup.-4 Torr the under-cut almost disappears, but the etching rate is 160 nm/min, which is too low and is not practical.
As mentioned above, in the conventional multilevel resist process, a mask pattern (a planarizing lower layer pattern) formed in accordance with a conventional multilevel resist process has an undercut portion, and this pattern is unsatisfactory for a fine and accurate patterning of the layer under the mask pattern.
From the point of view of preventing undercutting during an anisotropical etching process, when silicon, polycrystalline silicon or tungsten silicide (WSi.sub.x) is selectively and anisotropically etched, the etching gas includes carbon halide (e.g., CCl.sub.4, CF.sub.4 and the like) gas. The carbon halide gas is decomposed to form a carbon layer (i.e., adsorbed carbon) on sidewalls of, e.g., the polycrystalline silicon layer, to prevent the undercutting (lateral etching). This carbon layer is deposited on the exposed surface but is removed by ion bombardment. Nevertheless, the carbon sidewall protection can not be used for the anisotropical etching of organic material. In particular, if fluorocarbon (e.g., CF.sub.4) gas is added to the etching gas, the neutral active species of oxygen are increased, which tends to increase the undercutting of the organic material layer compared with the use of the etching gas of oxygen only.